Divergent selection for high or low growth rate modifies the response of muscle cells to serum or insulin-like growth factor-I in vitro.

Genetic differences in growth potential could result from changes in the levels of growth stimulatory factors or in the response of target tissues. The latter possibility was tested in adult myoblasts prepared from chickens selected for high (HG) or low growth rate (LG). Stimulation of [3H]-thymidine incorporation into DNA by serum was of higher amplitude in HG than LG muscle cells irrespective of whether the cell preparations were enriched in myoblasts or fibroblasts. HG myoblasts were also more responsive to insulin-like growth factor-I (IGF-I) in terms of [3H]-thymidine incorporation. IGF analogues with a reduced affinity for IGF binding proteins gave similar results suggesting that activity of binding proteins could not explain the difference between cells from the HG and LG lines. This difference was restricted to the proliferative stage because in myotubes, basal or IGF-I stimulated glucose and amino acid transports, tyrosine incorporation and protein degradation were not different.